The present invention pertains to the circuit housing art and, more particularly to a low microphonic circuit housing.
The need for a low microphonic circuit housing has been long felt, especially in the high frequency radio art, and particularly in the synthesizer field. There, for example, it is desirable to contain a very high frequency oscillator circuit, fabricated on a ceramic substrate, in a housing which isolates the circuit from external mechanical shock, thereby minimizing circuit microphonics. Microphonics are variations in the electrical performance of a circuit, for example, a change in the frequency of oscillation of an oscillator circuit, as a direct result from mechanical stress or vibrations imparted to sensitive components in the electrical circuit. This problem is especially acute at very high frequencies since any motion of a component in an RF field may frequency or phase modulate the RF circuit thereby inducing spurious modulation.
The prior art has developed two basic ways of minimizing microphonics in high frequency circuits. The first approach has been to use circuit components which are inherently immune to vibrational variations. In an oscillator, for example, it has been known that the use of crystals will greatly reduce the resulting circuit configuration from induced microphonics. However, this approach is not always commercially feasible due to the high cost of crystals.
A second, more common approach, is the use of microphonic sensitive circuit components in a totally encapsulated housing. Such an assembly normally includes a housing and cover portion which totally encase the circuit. An encapsulating material, such as epoxy foam, is used to completely fill the circuit casing thereby encapsulating all parts of the circuit within the casing structure. The resulting low microphonic assembly has the serious disadvantage that the circuit components are not accessible for adjustment or repair purposes.